The gross ingredient that glowworms use to make sticky snares

Survival is a sticky situation for many in the animal world. The glowworm is no exception. These fly larvae, native to Australia and New Zealand, catch dinner by using their bioluminescent blue "butts" to lure insects into a hammock of sticky threads.

Now, biologists have figured out what glowworms use to make these "fishing lines" — and it's a little gross. The main ingredient is urine.

Austrian zoologist Janek von Byern and his team made this discovery by collecting the fishing lines from Arachnocampa luminosa glowworms living in two caves on New Zealand's North Island. All nine species of glowworms light up due to a bioluminescent organ near their rear end. Unlike spider silk, glowworms spew their sticky threads from the mouths rather than the bum. But until now, the chemical and structural properties of glowworms' "fishing lines" had not been examined in depth.

Byern and company analyzed the physical and chemical components of the threads using scanning electron microscopes and X-ray spectroscopy. They found the moist threads are coated with mucus-like droplets that contain a couple of proteins, a greasy lipid and urea — the main component of insect waste and urine.

Arachnocampa larva attracts the prey with its light organ (LO) and then catches it with adhesive threads and droplets (v). Image courtesy the Royal Society of New Zealand.

The presence of urea suggests the fishing lines are somehow produced in the animal's gut, von Byern said, given the lines are cast, retracted and released again through the mouth.

"Excretion of most animals comes of out the back via the anal canal," said von Byern, whose work was published Wednesday in PLOS ONE. "But in glowworms, there is a light organ there, so all waste has to come out of the mouth part where the glue is produced. We guess the glue production is part of the excretion of the waste."

Glowworm snares are easy to find, hard to study

The glowworms aren't the first to rely on a urea-based adhesive. Humans have used urea and formaldehyde as wood glue in the past.

"So far, our guess is that urea and the protein together make the threads gluey or sticky," said von Byern.

The tests also revealed the threads and droplets are "hydroscopic," or attract water. Remove the fishing lines from the dank caves, and they dry out rapidly. In fact, their adhesive nature disappears when humidity levels drop below 80 percent.

As a result, many of the study's experiments were conducted within the caves themselves. The team even had to haul dry ice to the cave in order to freeze thread samples for their scanning electron microscope.

Based on these findings, von Byern argues that glowworm threads and spider webs are an example of convergent evolution — when similar traits evolve separately among different species.

Orb spiders, for example, spin their webs in dry environments — the opposite from the glowworms' cool, moist caves. The spiders spawn their silk from abdominal glands, while the glowworm threads come from their mouth. Finally, orb spiders' silk contains sugar, which is absent from glowworms' fishing lines.

Von Byern plans to compare the Arachnocampa luminosa threads with those from other fly larva species. He believes the research could one day lead to new forms of industrial or medical adhesives.

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